In relation to a conventional optical disc reproducing apparatus and the PRML processing, Japanese Patent Application Laid-Open Publication No. 2006-48737 (Patent Document 1) discloses technology of switching a PRML processing function according to a high-density recording region and a low-density recording region of an optical disk (paragraphs 0006, 0007 and the like).
And, Japanese Patent Application Laid-Open Publication No. 2004-178627 (Patent Document 2) discloses a Viterbi decoding circuit and an information reproducing apparatus in which a reference value (a reference level) for each path is generated at a reference-value generation circuit based on an inputted value and a generated information bit and is inputted as a reference value of a branch-metric calculation circuit (paragraphs 0023 and the like). Furthermore, it is described that by adaptively-processing the reference level of the branch-metric calculation, it becomes possible to follow fluctuation of a channel characteristic (paragraph 0023).
In the technology disclosed in Patent Document 1, in each of areas having recording density different from each other, a PRML class suitable for each area can be used respectively. However, this technology cannot support a case in which a characteristic, such as an amount of inter-symbol interference of a reproduction signal, is changed in an area of the same recording density in the same type of disks due to a factor other than change in the recording density.
And, in the technology disclosed in Patent Document 2, although the characteristics can be adaptively changed to a PR characteristic (a PRML processing) with constrained length (hereinafter referred to as k) of 4 (constant) as represented by PR(a, b, c, d), the characteristics cannot be adaptively switched to a PR characteristic with other constrained length, such as PR(a, b, c) (k=3) and PR(a, b, c, d, e) (k=5).
Increase in recording density of a disk causes problems in which desired effect cannot necessarily be obtained for signal reading accuracy at data reproduction by a conventional PRML processing with constant constrained length (k=4, for example). A first problem is that since a method of switching of PRML processing functions is fixed conventionally, an optimum PRML class (a PR coefficient) cannot necessarily be selected. A second problem is that since the PRML constrained length (k) is fixed conventionally, although a reproduction signal characteristic with a certain width can be supported, an error tends to occur for a wide-band signal when the PRML constrained length is fixed to a value corresponding to a narrow-band.